[PATCH] D74400: AMDGPU: llvm.amdgcn.writelane is a source of divergence

[Nicolai Hähnle via Phabricator via llvm-commits]

nhaehnle created this revision.
nhaehnle added reviewers: arsenm, foad, mareko.
Herald added subscribers: kerbowa, hiraditya, t-tye, tpr, dstuttard, yaxunl, wdng, jvesely, kzhuravl.
Herald added a project: LLVM.

Consider:

  %r = call i32 @llvm.amdgcn.writelane(i32 0, i32 1, i32 2)

This produces a value that is 0 on lane 1, and 2 everywhere else; i.e.,
it is divergent.

Reported-by: Marek Olsak <Marek.Olsak at amd.com>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D74400

Files:
  llvm/lib/Target/AMDGPU/AMDGPUSearchableTables.td
  llvm/test/Analysis/DivergenceAnalysis/AMDGPU/intrinsics.ll


Index: llvm/test/Analysis/DivergenceAnalysis/AMDGPU/intrinsics.ll
===================================================================
--- llvm/test/Analysis/DivergenceAnalysis/AMDGPU/intrinsics.ll
+++ llvm/test/Analysis/DivergenceAnalysis/AMDGPU/intrinsics.ll
@@ -42,12 +42,20 @@
   ret void
 }
 
+; CHECK: DIVERGENT: %tmp0 = call i32 @llvm.amdgcn.writelane(i32 0, i32 1, i32 2)
+define amdgpu_kernel void @writelane(i32 addrspace(1)* %out) #0 {
+  %tmp0 = call i32 @llvm.amdgcn.writelane(i32 0, i32 1, i32 2)
+  store i32 %tmp0, i32 addrspace(1)* %out
+  ret void
+}
+
 declare i32 @llvm.amdgcn.ds.swizzle(i32, i32) #1
 declare i32 @llvm.amdgcn.permlane16(i32, i32, i32, i32, i1, i1) #1
 declare i32 @llvm.amdgcn.permlanex16(i32, i32, i32, i32, i1, i1) #1
 declare i32 @llvm.amdgcn.mov.dpp.i32(i32, i32, i32, i32, i1) #1
 declare i32 @llvm.amdgcn.mov.dpp8.i32(i32, i32) #1
 declare i32 @llvm.amdgcn.update.dpp.i32(i32, i32, i32, i32, i32, i1) #1
+declare i32 @llvm.amdgcn.writelane(i32, i32, i32) #1
 
 attributes #0 = { nounwind convergent }
 attributes #1 = { nounwind readnone convergent }
Index: llvm/lib/Target/AMDGPU/AMDGPUSearchableTables.td
===================================================================
--- llvm/lib/Target/AMDGPU/AMDGPUSearchableTables.td
+++ llvm/lib/Target/AMDGPU/AMDGPUSearchableTables.td
@@ -247,6 +247,7 @@
 def : SourceOfDivergence<int_amdgcn_mov_dpp>;
 def : SourceOfDivergence<int_amdgcn_mov_dpp8>;
 def : SourceOfDivergence<int_amdgcn_update_dpp>;
+def : SourceOfDivergence<int_amdgcn_writelane>;
 
 def : SourceOfDivergence<int_amdgcn_mfma_f32_4x4x1f32>;
 def : SourceOfDivergence<int_amdgcn_mfma_f32_4x4x1f32>;


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